Near the end, the most crucial initiative of Mayor Julián Castro's rising political career was relegated to a tossup.

“I always figured it was going to be a close race,” Castro said, but the precariousness of Pre-K 4 SA almost certainly came as a surprise, particularly for a popular mayor who devoted months to selling it as a salve for San Antonio's most corrosive problem, its paltry levels of educational attainment.

When early votes had been counted, the proposition was up by only 87 votes, or 50 percent. That lead widened to 53 percent on Election Day.

Sources close to the campaign traced the slim margin to an unexpected source: not a backlash by too many Republicans, but rather a recoil by too many Democrats who weren't adequately educated about the unwieldy initiative.

With that wording, Castro could forget about winning tea party types. So the campaign, flush with cash, pushed to persuade a portion of right-leaning residents, unleashing an ad blitz on television a few days before early voting and targeting about 60,000 households with direct mail.

Castro sought to win such support by stressing that his plan transcended politics. Hoping to unite the city for a common good, he repeated this mantra up to Election Day, echoing Barack Obama's own keynote speech in 2004, when the future president sought to unite a nation.

“This is not a Democratic plan or a Republican plan,” Castro said. “This is a San Antonio plan that's in the best interest of all of our children.”

Yet as Obama's election in 2008 exposed bitter rifts among Americans, Castro's appeal to pragmatism also attracted partisan heat. Local Republicans, notably Bexar County Commissioner Kevin Wolff, argued against the initiative by slinging red meat: aversion to new taxes, distrust of government programs.

Another threat to the plan came from a constituency the campaign believed it could count on the most: Democratic voters.

An example is Gloria Flores, a woman with six grandchildren, the youngest a 2-year-old girl. Emerging Tuesday from a polling site near downtown, she said she'd voted for Obama but against Pre-K 4 SA.

“I don't think it's fair to all of the children, all of the families,” she said. “I just can't go with it with the way I understood it.”

The way Flores understood it, however, was incomplete.

She knew the program would be free for families that qualify for state-funded pre-K, including those at or below 185 percent of the federal poverty level. She did not know the program would reserve 10 percent of enrollment, at affordable rates, for families above that level.

Believing her middle-class family couldn't benefit, Flores rejected the measure. A broader effort to educate voters, occurring earlier in the campaign, might have taught her otherwise.

Of course, the most critical function of the plan is to lift the prospects of economically disadvantaged children, who make up 64 percent of those in local independent school districts.

Thousands of these children will now receive a boost in full-day pre-K. Learning alongside them, their middle-class schoolmates will benefit as well, in kindergarten and beyond. The entire city's economic future is brighter as a result.

The campaign overestimated the support of Democrats, but the initiative passed because enough voters, conservatives among them, grasped this interconnection.